The present invention relates to an actuating device such as the master cylinder for a vehicular hydraulic brake system with slip control, wherein a piston, which is preferably selectively subjected to the pressure of an auxiliary pressurized fluid, is slidably guided in a positioning sleeve, the positioning sleeve being adapted to be coupled with the piston in the direction of actuation and subjected to the auxiliary pressure in opposition to the actuating force.
A device including the characteristics described generally is known from the German printed and published patent application No. 30 40 561.9. This known brake system is comprised, essentially of a hydraulic power booster situated upstream of a master cylinder. In dependence upon the force exerted on the braking pedal, a dynamic pressure is created in the pressure chamber of the hydraulic power booster which is directed to a first brake circuit and displaces, the booster piston in the direction of actuation so that the working chambers of the master cylinder and/or the other brake circuits are pressurized as well.
At the pedal-remote end of the booster piston, an intermediary piston is formed, which is slidably guided within a positioning sleeve and which has a stop which abuts on the sleeve after a specific pedal travel, thus preventing a further axial displacement of the booster piston in the direction of actuation.
In the brake circuits of the brake system described above, electromagnetic valves are arranged, which are controllable through the intermediary of slip control electronics, and by means of which the braking pressure can be held constant or reduced in dependence upon the control signals of the control electronics. In such a control cycle, the pedal-remote annular surface of the positioning sleeve is simultaneously acted upon by the pressure prevailing in the booster chamber, the positioning sleeve being thus displaced in opposition to a pressure spring in the direction of the braking pedal. A reset of the braking pedal is achieved as soon as the positioning sleeve abuts against the stopo f the intermediary piston. In this manner, the master cylinder travel is limited to a certain value so that even in the event of a failure of the energy supply, a sufficient pressure fluid volume is always contained in the working chambers of the master cylinder, thus allowing easily for an emergency actuation of the brake system without power assistance.
The relatively large overall length of the brake system described above is not a preferable form, the overall length being in part due to the intermediary piston required, which is arranged between the booster piston and the master cylinder piston.